Zein, a mixture of water-insoluble proteins, is a valuable by-product of corn processing. To extract and purify zein requires a complicated train of steps, so costs are high, and zein in purified form currently sells for up to $10/lb or more. As a result, uses of purified zein are limited to specialty applications that can tolerate this price, such as pharmaceutical tablet coatings or confectionery coatings. Most zein remains unextracted and is simply carried over into comparatively low-value products, such as animal feed.
However, zein has many potential uses, including edible coatings for foodstuffs and biodegradable polymer resins, and a broader market for zein would improve the overall economics of corn processing.
U.S. Pat. No. 4,624,805, to Texas A&M University, describes a process for recovery of protein from a corn slurry, including an ultrafiltration step using Romicon PM30 membranes or the like.
U.S. Pat. No. 5,410,021, to Energenetics, Inc., describes recovery of various protein fractions from corn, including an ultrafiltration step using inorganic microfiltration membranes, such as aluminum oxide membranes.
U.S. Pat. No. 6,433,146 and U.S. Patent Application Publication 2002/0183490, both to the University of Illinois, describe an integrated process that can be optimized to maximize production of oil or zein. The process includes ultrafiltration, nanofiltration and pervaporation steps.
An article by N. Singh et al., entitled “Membrane Technology in Corn Wet Milling,” (Cereal Foods World, Vol. 42, No. 7, pp. 520-525, July 1997), includes a section describing techniques for protein recovery from corn gluten meal, including microfiltration and ultrafiltration steps.
Most ultrafiltration membranes, including those mentioned in the references cited above, are porous, and are subject to internal fouling by components of the feed solution. Internal fouling is usually irreversible and may cause the transmembrane flux to decline to an unacceptable level in a matter of weeks or months. At this point, the only solution is to replace the membranes.
A few attempts to use non-porous membranes as ultrafiltration membranes have been reported in the literature.
U.S. Pat. No. 5,145,584, to Allied Signal, Inc., describes use of a composite ultrafiltration membrane having a dense coating of a polyelectrolyte complex for separating glucose from higher molecular weight sugars.
An article by K. Ebert et al., “Solvent resistant nanofiltration membranes in edible oil processing,” (Membrane Technology, No. 107, p. 5-8, 1999), describes the use of composite membranes having a non-porous cellulose or polyether-polyamide block copolymer selective coating layer in experiments relating to separation of rapeseed oil from acetone. The reference also cites fluxes for pure 2-propanol and ethanol through both membrane types.
An article by S. Nunes et al., “Dense hydrophilic composite membranes for ultrafiltration,” (J. Membrane Science, Vol. 106, p. 49-56, 1995), compares the fouling resistance of composite membranes having a polyether-polyamide block copolymer selective coating layer with that of various commercial membranes. The membranes have molecular weight cut-offs between about 800 and 4,500. The experimental membranes showed fouling resistance as good or better than commercial membranes when tested with oil-in-water emulsions or with milk containing 3.5 wt % fat.
German Patent DE4237604, to GKSS, includes similar experimental details to those of the Nunes et al. paper. The patent covers the uses of a non-porous membrane made from a polymer having a polyether segment in the ultrafiltration treatment of aqueous solutions.
It is an object of the invention to provide a membrane filtration process for separation of zein from a solvent.
Additional objects and advantages of the invention will be apparent from the description below to those of ordinary skill in the art.